It is well-known that a venthole is provided in an airbag, and that when an occupant or the like of a vehicle hits the expanded airbag, an impact applied to the occupant or the like of the vehicle is absorbed by means of discharging a gas from an inside of the airbag via the venthole.
In Japanese Unexamined Patent Application Publication No. 2000-16228, an airbag constructed such that the venthole is covered by a lid member until gas-pressure in the inside of the airbag reaches a predetermined pressure, and when the gas-pressure exceeds the predetermined pressure, the lid member is pushed open by the gas-pressure and the venthole is opened is described.
Incidentally, in the same publication, an airbag for use in a driver's seat of a vehicle is exemplified. The airbag in the same publication is formed by stitching each of peripheral edge portions of respective two round shaped sheet members (panels), and the entire airbag is formed to be one chamber. The venthole is provided in the sheet member on an opposite side that is opposite to a vehicle-occupant side of the airbag, and a lid member is provided so as to cover the venthole.
In the airbag in the same publication, when the airbag is expanded, since the venthole is covered by the lid member until the gas-pressure in the inside of the airbag reaches the predetermined pressure, and the discharge of the gas from the venthole is limited, the inside of the airbag rapidly reaches high pressure and the airbag is rapidly developed.
Further, when the gas-pressure in the inside of the airbag exceeds the predetermined pressure, since the id member is pushed open and the venthole is opened, in a case that the vehicle-occupant hits the expanded airbag, the gas is discharged from the inside of the airbag via the venthole and thereby the impact applied to the vehicle-occupant is absorbed.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-16228
In the aforementioned Japanese Unexamined Patent Application Publication No. 2000-16228, there is a possibility that when the pressure in the inside of the airbag rises to the predetermined pressure or more, even when the stage is that before the occupant hits the airbag, the venthole is opened and the gas is needlessly discharged from the venthole.